Many machines and electrical installations can be controlled via control devices, especially via control devices like push-buttons or mode switches. Control devices are usually modularly constructed, including an actuator, a mounting part such as a ring nut or an assembly holder, so that they can be connected to one or more switching elements that are implemented as normally open contacts (make contact elements) or as normally closed contacts (break contact elements). A control device and a switching element may then be installed into a control panel or a control board, for example.
An actuator is usually installed into a control panel or control board by bringing it through an opening on the front side of the control panel or control board and then installing it from behind by using a mounting part. The switching elements are connected to the actuator or to the mounting part mechanically, such as with screws, snap-on mounting or latches. The electrical connection between the switching elements and the control system is then made over the connecting terminals.
In security applications, such as in emergency off control devices, standard specifications may make it necessary to generate a control signal by using contacts with positive opening operation. Thus, in a non-actuated emergency off control device, the contacts and thereby the corresponding current circuit are closed. In case of a disturbance or if necessary, by actuating the emergency off actuator, usually located in front of the control panel or control board, a normally closed contact is opened and the electrical installation or machine has to be brought to a secured state. The opening of the normally closed contact is possible only if a correct spatial arrangement between the actuator and the switching element has been ensured. Because of improper installation or exposure to impacts, it may happen that a switching element has been mechanically separated from the actuator. In such a case, the emergency off control device does not operate properly any more, meaning that by activation if necessary, the normally closed contacts are not opened any more and the elimination of security risks cannot be guaranteed. This can lead to fatal damages to individuals and machines. This error cannot be detected by signaling, since if the switching element is spatially separated from the actuator, the normally closed contacts remain closed.
Furthermore, since switching elements are usually located behind a control panel or a control board or in a housing, they are not visible and therefore cannot be detected by the technical staff using the machine or installation. In particular applications, a switching element cannot be mechanically secured to the actuator. This would be the case if in a housing the switching element and the wiring were mounted onto floor (e.g. on a top-hat rail) and the actuator were fastened at the cap. Here that the spatial arrangement between the actuator and switch would be proper depends on whether or not the cap has been closed properly. If in an actuated emergency off control device the cap of the housing or the switch panel door is opened, the normally closed contacts that have been opened by actuating the control device are closed again and the emergency off command is thus overridden, which may have adverse consequences especially if this happens together with a further error.
Published U.S. Pat. No. 6,198,058 and German patent 4,101,493 both disclose switching systems for identifying or avoiding the error discussed above. The systems disclosed in them include normally closed contacts and normally open contacts. Here the switch slide is adapted, if the switching elements are properly installed to the actuator, to close the normally open contact, but not that much that the normally closed contact would be opened. If the normally open contacts and the normally closed contacts are installed in series, and if the spatial arrangement of the switching elements and the non-actuated actuator is appropriate, the current circuit is closed.
By actuating the emergency off control device, the normally closed contact is opened and the current circuit will be broken despite the still closed normally open contact. If the switching element is, for any reason, disengaged, the partly pre-stressed switch slide moves because of the spring force into its initial position and the current circuit is broken because the normally open contact is opened.
The English abstract of JP 12003-2722468 discloses a mounting mechanism for a control device. The switching element mounted on a support is mounted to the actuator by way of a turn-lock fastener. The turn-lock fastener is adapted to open the normally closed contact in all positions except the interlocking position. If the turn-lock fastener is in the interlocking position so that the switching element is not properly connected to the actuator, the normally closed contact remains closed, even though the spatial arrangement of the switching element and actuator is not correct. Thus, it cannot be guaranteed that the corresponding switching element is really ready for operation if the emergency-off current circuit is closed.
The English abstract of JP 12003-303527 discloses a switch slide that, carrying a movable contact member, is connected in a form-locking manner to the plunger of the actuator. If this contact is missing, the normally closed contact is opened by a spring that applies a force on the switch actuator in a direction opposite to the movement. Similar kind of functionality is used also in EP 1 1153 609 A1, whereby the connection between the switch actuator and plunger is obtained by way of a bayonet joint.